1. Field of the Invention
This invention relates to a diagnostic system for detecting leakage of fuel vapor from a purge system installed between an intake manifold and a fuel tank.
2. Description of Related Art
In order for gasoline engines, in particular automobile engines, to prevent release of either liquid fuel or fuel vapor into the atmosphere, an evaporation control system or a purge system is installed to collect fuel vapor. This evaporation control system is basically comprised of a canister and a purge valve disposed between an intake manifold and a fuel tank. Fuel vapor from the fuel tank is adsorbed in the canister and purged with negative intake pressure into the intake manifold from the canister.
Leakage of fuel vapor from the purge passage leads to evaporative emission into the atmosphere. In view of this point, it is important to detect leakage of fuel vapor from the evaporation control system. One of leakage detection techniques is known from, for instance. Japanese Unexamined Patent Publication No. 6- 74106. The prior art leakage detection is made based on a rising rate of the pressure in the purge passage lowered below the atmospheric pressure level and air-tightly closed. A problem of the prior art leakage detection is that a change in pressure is hardly understandable whether it is caused due to leakage or due to an increased amount of fuel vapor in external circumstances that cause an increased yield of fuel vapor. Consequently, the prior art leakage detection is interrupted in such external circumstances.
Japanese Unexamined Patent Publication No. 4- 362264 discloses a system for diagnosing a fault of a purge system under the condition that the engine remains below a specified temperature immediately after a start of engine operation, in other words, in a state where liquid fuel in the fuel tank is free from evaporation due to a rise in temperature of the liquid fuel.
It is practically difficult to directly detect whether fuel vapor is increasing at a high rate. Consequently, whether or not the rate of an increase in the amount of fuel vapor is high is indirectly detected by the utilization of a parameter relating to an increase in the amount of fuel vapor. For this reason, in view of preventing the leakage judgement from resulting in errors, the importance is to establish such a detectable state of the fact that the amount of fuel vapor increases and to provide sufficiently frequent opportunities to make the leakage judgement.